c++ - Why int array[][20]?

In ReadVector() why do I have to write int a[][20]? What is the purpose of [20]. Why can't I write a[][]?

int a[20][20 ], n,m;

int ReadVector(int a[][20],int n,int m){
for (int i=0; i        for (int j=0; j            cout<<"a["<            cin>>a[i][j];
            }

 return *a[20];`